Both carbon dioxide-based chromatography as well as some forms of supercritical fluid chromatography (SFC) use carbon dioxide as the mobile phase. Generally, it is desirable to keep the carbon dioxide condensed (e.g., liquid, at or near a supercritical state) throughout a separation. To maintain the desired state, the carbon dioxide is usually held under elevated conditions (e.g., elevated pressure). Carbon dioxide can change phase from supercritical or liquid to gas within such systems when the elevated conditions are not maintained.
A change in state can occur, for example, in a system designed to receive carbon dioxide from a tank and/or in a system subject to adiabatic heating or other heat transfer. When the carbon dioxide content of the tank is low, the tank cannot apply an adequate elevated pressure to maintain the carbon dioxide in the tank, or delivered to the system, in the desired state. Likewise, adiabatic heating of carbon dioxide during pumping or transport of the pressurized fluid in the system can also lead to phase changes. For example, supercritical/liquid carbon dioxide can undergo a phase transition to a gas or vapor state as a consequence of heat transfer from a pump head to the carbon dioxide mobile phase.
Changes in phase during a chromatographic and/or preparative chromatographic separation can lead to operational errors and/or undesirable outcomes such as irreproducibility, failed separation, peak distortion, and interfering noisy baseline signals.